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Myopia, also known as nearsightedness or short-sightedness, is a refractive defect of the eye, where the person affected usually can see nearby objects clearly but distant objects appear blurred. The opposite of myopia is farsightedness or hyperopia.

Myopia is the most common eyesight problem in the world. About one quarter of the adult population in the United States has myopia. In places like Japan, Singapore and Taiwan, as many as one in three or one in two of the adult population is myopic.

Myopia is measured in diopters; specifically, the strength of the corrective lens that must be used to enable the eye to focus distant images correctly on the retina. Myopia of 6.00 diopters or greater is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment or glaucoma. They are also more likely to experience floaters.

The most widely held theory of the cause of myopia is that it is mainly hereditary. Measures of the heritability of myopia have yielded figures as high as 89%, and recent research has identified geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are muchs that may be responsible: defective versions of the PAX6 gene seem to be associated with myopia in twin studies. Under this theory, the eye is slightly elongated front to back as a result of faults during development, causing images to be focusThe word focus (pl. foci has several meanings: focus (geometry) focus (optics) See also: autofocus, focal length focus (linguistics) Focus (band) focus (computing) Focus software Ford Focus (automobile) See also focal point (another disambiguation page).ed in front of the retinaeye cross-sectional view. Courtesy NIH National Eye Institute. Many animals have eyes different from the human eye. The retina is a thin layer of cells at the back of the eyeball of vertebrates and some cephalopods; it is the part of the eye which transdu rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescenceA separate article is about the punk band called The Adolescents. Adolescence is the transitional stage of development between childhood and full adulthood, representing the period of time during which a person is biologically adult but emotionally not at and then levels off as a person reaches adulthood.

Mainstream ophthalmologists most commonly correct myopia through the use of corrective lensA corrective lens is a prosthetic lens worn on or before the eye, used to treat myopia, hypermetropia, presbyopia and astigmatism. The most common types of corrective lens are spectacle lenses and contact lenses. Myopia (short sightedness) requires biconces, such as spectacles or contact lensA contact lens is a corrective or cosmetic lens placed on the cornea of the eye atop the iris. The idea of applying a corrective lens directly to the surface of the eye was first proposed and sketched by Rene Descartes in 1636, but in was not until 1887 tes. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak. Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. Other theories suggest that the eyes become strained by the contant extra work involved in "nearwork" and get stuck in the near position, and eye exercises can help loosen the muscles up thereby freeing it for far vision.

Regardless of the accuracy of the ciliary muscle theory, it should be understood that even a high heritability of myopia (as for any other condition) does not mean that environmental factors and lifestyle have no effect on the development of the condition. High heritability simply means that most of the variation in a particular population at a particular time is due to genetic differences. If the environment changes - as, for example, it has by the introduction of televisions and computers - the incidence of myopia can change as a result, even though heritability remains high.

One 2002 study suggested that myopia may be caused by over-consumption of bread in childhood. So far, this has not been supported by further research results.